The present invention relates generally to agricultural implements, and more specifically to hydraulic lift structure for such implements.
Agricultural implements with relatively large transversely extending frames typically include spaced lift wheel assemblies controlled by hydraulic cylinders. The lift wheel assemblies must be timed and remain generally in phase for level lift operation. Many implements include transversely extending rockshafts connecting the wheel assemblies for operation in unison. Various hydraulic circuit arrangements are available to operate the rockshafts and limit torsional wind-up of the rockshafts as uneven loads are encountered across the width of the implement frame. In implements wherein lift and/or gauge wheels are widely spaced at varying locations relative to the implement frame, provision of an economically feasible wheel lift and timing system is a continuing challenge. Often, there simply is insufficient space for mechanical timing structures such as rockshafts.
Series cylinder circuits such as shown in U.S. Pat. No. 3,627,053 have been widely used to provide level lift features. Such circuits often require rephasing cylinders or special rephrasing valves and matched cylinders, and large reaction forces are encountered by cylinders. Other circuits such as shown in U.S. Pat. No. 4,821,806 include valve and conduit structure providing parallel cylinder operation while permitting the operator to change to a fast raise/fast lower series mode wherein flow control structure causes the lift cylinders to operate substantially in series for raising and lowering the implement in level fashion across its width to and from a transport position.
In another type of structure, such as shown in U.S. Pat. No. 5,957,218, lift cylinders are individually controlled by a computer or other electronic control arrangement. This type of control is complex and costly and usually requires feedback transducers. A relatively simple and inexpensive lift system which is easily adaptable to placement of lift wheel assemblies at different locations on an implement frame without need for rockshafts for timing is needed.
It is therefore an object of the present invention to provide an improved lift wheel structure for an implement. It is a further object to provide such a structure which overcomes most or all of the aforementioned problems.
It is a further object of the invention to provide an improved lift structure for an implement which is relatively simple and inexpensive. It is a further object to provide such a structure which facilitates mounting of lift wheels at widely spaced locations on the frame. It is still another object to provide such a structure which does not require a rockshaft connection between spaced wheel assemblies to maintain timing of the assemblies.
It is a further object to provide an improved lift wheel structure for an implement which facilitates a simple parallel connection of lift cylinders. It is a further object to provide such a structure which maintains cylinder timing even under large differential loading of the cylinders. It is another object to provide such a structure having an automatic lock-up function so that separate lock-up valves are obviated.
To prevent the wheels of spaced lift wheel assemblies from getting out of phase, a hydraulic system includes a parallel circuit with pilot operated check valves on the base or lift end of each double-acting cylinder. The check valves lock hydraulic fluid into the base end of each cylinder and prevent uneven loads from changing the relative extension of the cylinders. The pilot is operated off the rod or lower end of each cylinder. Therefore, when the machine is lowered by applying fluid pressure to the rod ends of the cylinders, the check valve opens and allows oil flow out of the base end. An orifice is used to provide the optimum breakoff pressure for the check valve.
The hydraulic system eliminates need for costly rephasing cylinders and avoids the high reaction forces of a traditional series circuits. If desired, the hydraulic lock-up valve normally used during storage and machine maintenance can be eliminated because the check valves provide the same function.
These and other objects, features and advantages of the present invention will become apparent to one skilled in the art upon reading the following detailed description in view of the drawings.